A symmetrically round object such as a brake drum has two similar primary vibrational modes displaced 45.degree. away from one another. If the difference in frequency of vibration between these two modes is very small, a brake drum will be more likely to howl or squeal when the brakes are applied. It has been discovered that this problem, sometimes referred to as an instability problem, can be alleviated if the difference in frequency of these two modes can be increased to over about 50 Hertz (Hz). Hz is a measure of vibration frequency in cycles per second.
It has been proposed to solve this instability problem by adding symmetrically placed stabilizing sections around the periphery of the brake drum. The stabilizing sections add mass but also increase stiffness considerably. If four stabilizing sections are oriented at 90.degree. intervals, they will affect one of the two similar primary modes of vibration much more than the other and thus separate the vibration frequency of one mode relative to the other sufficiently to solve the instability problem. However, the stabilizing sections, while solving the instability problem, cause another problem and that is, as the brake shoes expand to apply the brakes, the weaker sections of the drum between the stabilizing sections deflect more than the stabilizing sections, distorting the brake drum. Accordingly, the frictional force at the stabilizing sections will be higher than that between the stabilizing sections, producing a variation in brake torque four times per revolution. Also, where the frictional force is higher, there is local heating (hot spots) which distorts the drum even more. This produces roughness and noise, both of which are objectionable.
What is needed is a brake drum construction which deals with the noise instability problem without causing distortion of the brake drum.